The quality attributes of fresh fruits, vegetables and cut flowers must be maintained as much as possible for as long as possible to ensure consumer acceptability. Quality deterioration of horticultural produce comes about through plant tissue enzyme reactions including respiration, ripening and senescence, through microbial growth and through water loss for the tissue. Methods of inhibiting the deteriorative enzyme reactions, and the growth of yeasts, molds and bacteria involve the reduction of the produce temperature to between 1.degree. and 12.degree. C., and the creation of low O.sub.2 /high CO.sub.2 modified atmosphere (MA) around the produce. Water in fruits and vegetables can be lost readily under low relative humidity conditions with the consequence of skin wrinkling, wilting and reduction in crispness. The rate of water loss from produce can be restricted by storing the produce in closed package systems consisting of walls with low moisture permeability.
Modified atmosphere packaging (MAP) of fruits and vegetables is a process involving:
1. Pre-packaging conditions and treatment of produce; PA0 2. The packing of produce into a gas-permeable package system; PA0 3. The introduction of predetermined CO.sub.2 and O.sub.2 containing gas mixture flush into the headspace of the package system or the retention of existing air in the head-space of the package system; and PA0 4. Closure and sealing of the MA package system. PA0 1. Maintain definitive beneficial equilibrium levels of CO.sub.2 and O.sub.2 in the headspace; PA0 2. Obviate gas pressure build-up within a package system; PA0 3. Minimize moisture loss from produce; PA0 4. Prevent produce crushing and bruising; and PA0 5. Inhibit water migration from the package interior into the walls of the package system for the purpose of retaining structural strength of the walls.
During storage, the fruits and vegetables convert O.sub.2 from the headspace to CO.sub.2 through the respiration process with the result that the O.sub.2 content in the headspace decreases while the CO.sub.2 content increases. The objective in the design of an effective package system for MAP of produce is to regulate the influx of O.sub.2 to an efflux of CO.sub.2 from the package headspace to achieve and maintain a suitable equilibrium modified atmosphere in the headspace around the stored produce for optimum retention of the quality attributes and for the reduction of microbial growth.
Low O.sub.2 levels and elevated CO.sub.2 levels in the headspace around a horticultural commodity reduce the respiration and ripening rates, and the growth of spoilage organisms (spoilogens). Unsuitable modified atmospheres around produce in a package system can induce physiological damage, prevent wound healing, enhance senescence and cause off-flavour formation. O.sub.2 levels lower than 1% bring about anaerobic respiration and off-flavour development, whereas CO.sub.2 levels of about 10% or higher inhibit spoilogen growth but may cause tissue damage to CO.sub.2 -sensitive commodities.
Package systems for MAP must be designed and constructed with specific packaging materials to meet the following requirements:
The gas and moisture permeabilities of package components of MA package systems are critical parameters. The technology of plastic polymeric films has advanced to such an extent that a specific gas permeability requirement can be met with a single plastic film or a multifilm combination, with or without vent pinholes.
In 1960, Eaves (J. Hort. Sci. 37:110, 1960) reported the effectiveness of gas-permeable, flexible polymeric barrier film as a package system for extending the life of fresh commodities. Tomkins (J. Appl. Bacteriol. 25:290, 1962) used polymeric film-covered trays as package systems to determine their effectiveness in establishing equilibrium MA around apples. Prior art on the use of bags made from polymeric plastic, gas permeable film such as polyethylene and polyvinylchloride, for prolonging of shelf-life of stored fruits and vegetables, is exemplified by U.S. Pat. No. 3,450,542, Badran, U.S. Pat. No. 3,450,544, Badran et al., and U.S. Pat. No. 3,798,333, Cummin. A more complex package system has been described by Rumberger (U.S. Pat. No. 3,630,759) in that an inner plastic pouch containing the produce is enveloped by an outer pouch containing an atmosphere of less than 15% O.sub.2. Both pouches are to be constructed from gas-permeable films.
Corrugated paperboard boxes and cartons are used commercially for the storage and transport of fresh horticultural commodities. The advantages of the corrugated paperboard boxes and cartons are the relatively low cost per unit volume, low thermal energy conductivity, impact absorbing ability to prevent bruising of commodities and ease of disposal at the receiving end.
However, conventional corrugated paperboard has a very high gas and moisture permeability and as such is unsuitable for modified atmosphere packaging of horticultural commodities.